Actually, some of the best players in the game do it at both ends of the court. Those are that are great offensive players but lousy defensive players also happen to play on some of the worst teams in the league (see Monta Ellis, Kevin Love, etc.).

In addition, it's not just Bargnani's defence that's the problem. At 7-feet tall and 250 pounds, you would expect a player of his size to grab more than 6 rebounds a game. At the very least, he should be grabbing 8 to 9 boards a game, which shouldn't require much effort for a guy of his stature, skill, and for the playing time he receives.

The other thing that's frustrating with Bargnani is that it seems that he's not giving his full effort at times on the court. He has moments where he just goes numb and becomes unfocused. Hopefully this changes as he matures, as he's still only 25 years old.

Finally, my other frustration with Bargnani is his conditioning. Specifically, he, like the rest of the Raptors, is dreadful on the 2nd of back-to-back games. Bargnani needs to get into better shape, and I hope Casey makes this one of his priorities in the off-season (assuming there is one).

To be honest, he grabs 6 rebounds because he plays high-post on D and around the arc on O. his location on the court will be more like a SF.
Time to guide this kid once and for all. If he doesn't get any better, trade him.